An analysis of the changes in growth rate occurring during the initial stages of geocurvature in shoots

Abstract A detailed study of the changes in growth rate on the two sides of a shoot prior to, and following geostimulation has been undertaken. Data from etiolated Zea seedlings, and cucumber hypocotyls, and from light grown sunflower hypocotyls are presented. In all cases, the differential growth which brings about geocurvature begins simultaneously along the length of the organ. This is contrary to previous reports in the literature which suggested that curvature begins first at the tip of an organ and progresses basipetally. The feature which is common to all species investigated is that growth of the upper side of the shoot ceases following geostimulation. In some cases there is also a marked acceleration in growth rate on the lower side of the shoot (sunflower) but other species show no such acceleration of growth (cucumber). It has been assumed for many years that the major factor causing upward curvature was an acceleration of growth on the lower side of the organ. The data presented here show that the cessation of growth on the upper side is a major, and in some cases the only factor bringing about geocurvature. The data are discussed in relation to the mechanisms which might control geotropic curvature.